There are several methods to make skins for plastic articles, for example skins on an automotive interior article, such as an instrument panel. Positive thermoforming, negative thermoforming, sprayed polyurethane (PU), and shush molding are four major processes used to make skins for such interior articles. Slush molding and sprayed PU offer the most design freedom with some process advantages. The overall system cost for slush molding is much lower than sprayed PU.
In the slush molding technique, a free-flowing, powdered polymer is charged to an open top container or box, i.e., a slush box. A heated mold in the form of the article or object to be molded is clamped on top of the slush box, and the container rotated in a manner such that the free-flowing polymer powder contacts the hot mold where the powder melts and flows over the mold. The container is then returned to its original position, the mold removed from the container, and the article removed from the mold. This technique can realize complex shapes with sharp edges and excellent grain retention.
Introduction of passenger and door airbags has changed the requirement for automotive interior skins from predominately appearance only criteria to that of a safety composition. Until recently, polyvinyl chloride (PVC) resins were the material of choice for interior skins, and they are ideally suited for slush molding. However, PVC formulations suffer from migration and volatilization of the plasticizers over time, and this leads both to physical property changes in the PVC as it ages and to fogging of the car window glass. PVC also suffers from being heavier than alternative materials (an important consideration in the current design of automobiles with the emphasis on lighter materials to reduce the overall weight of the vehicle and thus increase its gas efficiency). Additionally, the hardness, storage modulus, and brittleness of PVC increases as the ambient temperature decreases, and thus at low temperatures, e.g., about −40° C., the instrument panel skin upon airbag deployment could splinter.
An alternative to PVC is thermoplastic polyurethanes (TPU) which can be engineered to have the necessary flow characteristics required for slush molding. TPU has good scratch and mar properties and better low temperature properties than PVC, but aromatic based TPU has poor ultraviolet (UV) light resistance. Aliphatic isocyanates can be used to prepare TPU having good UV light resistance but at a significant cost penalty.
Blends of polypropylene (PP) and a polyolefinic rubber, referred to as thermoplastic polyolefin (TPO), is yet another alternative. TPO posses better ductility than PVC. Moreover, it retains its ductility over time since it does not contain any low molecular weight plasticizers, as does PVC. TPO performs better in comparison to PVC in interior automotive skin applications. TPO is less expensive as compared to TPU.
A critical property for good slush molding moldability is powder flow for achieving good surface quality in terms of good grain appearance and good grain definition. However, conventional TPO require cryogenic (subzero) pulverization which may create hooks and tails which adversely impacts the powder flow of slush TPO and, hence, the quality of the part, see U.S. Pat. No. 7,037,979 and US Publication No. 2004/0147680, both of which are incorporated herein in their entirety. Additionally, cryogenic pulverization adds complexity and cost to manufacturing a powder TPO. Ambient temperature pulverizing has been disclosed with the addition of a grinding aid. For example, U.S. Pat. No. 4,650,126 discloses the addition of such grinding aids as silica, calcium carbonate, zinc oxide, magnesium oxide, clay, and the like. Another example of ambient grinding with a grinding aid is taught in U.S. Pat. No. 6,803,417 wherein an exotic silane-grafted multi-component TPO composition which requires curing has been disclosed, however at a significant cost penalty.
Thus, there is a need by the automobile manufacturers and others to develop a polymer composition for automotive interior applications, especially a polymer powder for slush molding operations, having adequate hardness (e.g., Shore A hardness) with the ability to be pulverized at ambient temperature or above and having improved flow and sintering characteristics.